1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling a nut runner which tighten a fastener such as a bolt, a nut, or the like by reducing stepwise the rotational speed of the output shaft of the nut runner.
2. Description of the Background Art
Nut runners are employed to tighten fasteners such as bolts, nuts, or the like. When tightening such a fastener with a nut runner, in order to reduce the time required to tighten the fastener, the fastener is first tightened with a low torque and at a high speed until the tightening torque reaches a prescribed level from the start of tightening the fastener. Then, when the prescribed torque level is reached, the fastener is tightened with a higher torque at a low speed. Such a two-stage tightening process is performed by rotating the output shaft of the nut runner first at a higher speed and then at a lower speed.
One known fastener tightening process of the above type is disclosed in Japanese Patent Publication No. 53-3840, for example. In the disclosed process, the fastener is tightened at a high speed by a nut runner driven by a DC motor until a prescribed torque level is reached.
The current supplied to the DC motor and the torque produced by the DC motor are proportional to each other. Therefore, the torque of the DC motor can be detected by detecting the value of the current supplied thereto. When the torque thus detected of the nut runner output shaft reaches a certain switching torque level, the power supply that energizes the DC motor is switched from a higher voltage unit to a lower-voltage unit to lower the voltage applied to the DC motor, thereby rotating the nut runner output shaft at a lower speed. When the torque of the nut runner output torque, as detected by a separate torque sensor, reaches a prescribed tightening torque level, the DC motor is de-energized completing the tightening process.
In the above tightening process, the rotational speed of the motor is switched from a higher speed to a lower speed immediately before the fastener, such as a bolt, is seated on a surface. Since the tightening torque of the output shaft increases abruptly just before the fastener is seated, however, the rotational speed of the motor may not appropriately be switched from the higher speed to the lower speed on account of an error in the motor current measured for torque detection, a fluctuation in the rotational speed of the motor, a delay in the response to the switching between the motor speeds, an inertial force of a rotating member such as the output shaft, and other factors. If the motor speed is switched from the higher speed to the lower speed after the fastener is seated, then the tightening torque is increased in excess of the desired torque level due to the response delay in the switching from the higher to the lower speed. Particularly, inasmuch as the fastener has been tightened at a high speed immediately before it is seated, the inertial force of the output shaft is large, and the fastener tends to be excessively tightened due to such large inertial force even after a switching signal to change the motor speed from the higher speed to the lower speed is issued. Such a switching signal may be applied earlier to change the motor speed from the higher speed to the lower speed in order to avoid any excessive tightening of the fastener. Then, the motor speed may be switched to the lower speed too early because of a measured motor current, a motor speed fluctuation, or the like, with the result that the fastener may be tightened at the lower speed for a certain period of time before it is seated, and the total tightening time required may be prolonged.
To prevent the above problems, it is necessary to lower the rotational speed of the nut runner output shaft to reduce the inertial force thereof when the rotational speed is high. Typically, the rotational speed of the output shaft is reduced to about 300 rpm. Therefore, efforts to reduce the total tightening time are limited.
U.S. Pat. No. 3,965,778 discloses a procedure for reliably switching the rotational speed of the output shaft of a nut runner from a higher speed to a lower speed. According to the disclosed process, when a prescribed tightening torque is detected at a snug point immediately before a bolt or the like is seated on a surface, the motor is deenergized to stop the rotation of the output shaft so as to maintain an output shaft torque corresponding to a fastener seating torque.
However, since a torque increase from the tightening torque at the snug point to a final tightening torque is extremely abrupt, a certain response delay may nevertheless occur and the fastener may still be excessively tightened when the output shaft is stopped at the final tightening torque level.